Central Isthmus Watershed Study

Final Report

The Central Isthmus Watershed Study Final Report summarizes how the flood model was built, existing flood risk, and the proposed conceptual solutions. 

Project Overview

The City of Madison completed a watershed study in the Central Isthmus watershed (as shown below). The City of Madison contracted with Tetra Tech to complete the study.  The watershed study identified causes of existing flooding and then looked at potential solutions to try to reduce flooding.  The study used computer models to assist with the evaluations. 
For more information please see the Flash Flooding Story Map . *Note: Please view the story map using Firefox or Google Chrome browsers. Story maps are not viewable with Internet Explorer.

The Isthmus Central watershed (shown in light blue) drains to Lake Monona through several storm sewers.

Still have questions? Get some answers here from our additional information page on the Watershed Studies.

Project Schedule

The study began in October 2021 and the assessment was completed at the end of 2022.

Public Engagement

Questions and comments can be emailed to the Project Contact at anytime.  There were also a number of points of contact during this project where the public was encouraged to give feedback. Information is summarized below:

Public Information Meetings

Sept. 1, 2022 Public Information Meeting Recording

Sept. 1, 2022 Public Information Meeting PowerPoint Presentation

Focus Group Meetings

Due to the shorter duration of this watershed study, formal Focus Group Meetings were not conducted. The City received a lot of feedback and met with residents in known flooding areas and had a strong understanding of the existing issues. The public is encouraged to provide feedback directly to the project manager, and to continue to report flooding online—see below.

Report Flooding

Report flooding and damage to private property or public lands in the City of Madison. This information is used to improve the stormwater system and to reduce flooding damage in the future. If you would like more information on creating a comprehensive report, please review our Flood Reporting Tips. 

Existing Conditions Mapping

To share the existing conditions mapping developed during the watershed study, the City developed an online Watershed Study Flood Risk Map. This map shows detailed flood depths for a storm that has a 1% chance of occurring annually, which is 6.66 inches of rain in 24 hours.

Proposed Flood Reduction Solutions

The watershed study identified many flood reduction solutions. These solutions are concepts and show what it would take to meet the City's flood reduction targets. Only solutions included in the Capital Improvement Plan budget are in process of design or construction and public outreach efforts are conducted for those projects individually, and separately from this study.

Details on the specific solutions that were identified can be found in the final report linked at the top of this page, or in the latest Public Information Meeting presentation linked above. 

Background

A watershed is a bounded area of land that drains rain and snow to the same point. This point is frequently described as the outlet of the watershed. Watersheds are defined on many different scales, so they can vary greatly in size. A watershed can be as small as the part of your roof that drains to one downspout or be as large as the land in North America that drains to the Gulf of Mexico via the Mississippi River.

The City of Madison has identified 24 watersheds within the City, each defined by its unique stormwater drainage system (the drainage ditches, pipes, ponds, and greenways that convey stormwater) and a common outlet point (frequently a specific point along Lake Mendota, Lake Monona, or Lake Wingra shoreline). Note: some of the smaller watersheds have been combined into one study area and report.

The oldest of these drainage systems dates back to the 1880s, when there were very few, if any, standards guiding their design. As the City developed, so did the standards for design and construction. Today, we have comprehensive City, State, and Federal regulations to guide design and construction. However, each neighborhood was developed with the regulations that were set at that time, not with today's regulations. You can view this StoryMap to see the age of the infrastructure in your neighborhood.

On Aug 20, 2018, Madison and the surrounding areas experienced a historic rain event which resulted in extreme flash flooding, subsequent lake level rises and associated Isthmus flooding. This flooding revealed widespread deficiencies in the City's stormwater drainage systems. Since that event, the City's Stormwater Utility has been developing new initiatives to create a more resilient system. One of the key initiatives, launched in January 2019, is the City of Madison Watershed Study Program. The main goals of the Watershed Study Program are to:

1. Understand the existing stormwater system limitations and the current level of flood risk across the City
2. Identify conceptual projects that help mitigate the existing flood risk

These goals are being achieved through 20 City of Madison Watershed Studies that cover all 24 watersheds that were identified. 

During each study, a computer model is being constructed and calibrated, using real world data, to try to simulate the risk of future flood events. These models are then also used to test a variety of hypothetical projects to identify ones that mitigate flooding while being a good fit for the neighborhood. 

With each Watershed Study that is completed, the City adds to its long list of potential flood projects that have been identified. These projects include both simple storm sewer system upgrades and large major flood mitigation projects (ex: Mendota Grassman Greenway). Major flood projects are expensive, so they are constructed over time as budget allows. Typically, the Stormwater Utility has about $4-$5M per year to address flooding in various types of projects throughout the City. The cost of just the major flood projects identified so far totals over $620M. Implementation of these flood mitigation measures will take many decades.

To equitably prioritize and budget projects, the City analyzes data that combines several factors that were prioritized with resident input. These factors include whether a project:

• Increases access for emergency vehicles
• Provides flood protection for homes that flood most frequently
• Provides flood protection to vulnerable populations
• Also improves stormwater quality

For more details on the City of Madison Watershed Study Program please visit the Watershed Study Webpage and the Watershed Study Learning Hub.

Additional Resources

• City of Madison Flood Website
• View the City's Annual Stormwater Newsletter: Waterways
• Flash Flooding Resilience Story Map *Note: Please view the story map using Firefox or Google Chrome browsers. Story maps are not viewable with Internet Explorer.
• Watershed Studies 2019 Audio Presentation
• Flood Prevention Flyer and website
• LISTEN: Everyday Engineering Podcast Episode: Basement Drainage(external)
• LISTEN: Everyday Engineering Podcast Episode: Historic Flooding(external)
• LISTEN: Everyday Engineering Podcast Episode: What's going on with the Watershed?